The present invention relates to a device for driving an induction motor.
The conventional device of this type which has been put into practice can be represented by a circuit system which is shown in FIG. 1, in which reference numeral 1 denotes a variable converter which converts an alternating current into a direct current. This converter is, for example, composed of a normal thyristor converter. Numeral 2 denotes a D-C reactor, numeral 3 denotes a smoothing capacitor, numeral 4 denotes an inverter which converts a direct current into an alternating current. This inverter is, for example, composed of a normal thyristor converter. Numeral 5 denotes an induction motor, numeral 6 denotes a speed generator, numeral 11 denotes the output of the speed generator, numeral 12 denotes a speed controller, which is, for example, composed of a normal operational amplifier. Numeral 13 denotes the output of the speed controller, numeral 14 denotes an adder for adding the speed generator output 11 and the speed controller output 13 together, and numeral 15 denotes the output of the adder 14.
The operation of the conventional device of FIG. 1 will be described here below. Under ordinary operating conditions, a reference speed signal and the speed generator output 11, which are applied to a first and second inputs of the speed controller 12, respectively, will be equal to each other. The adder 14 adds the output 13 of the speed controller 12 and the speed generator output 11 and the output 15 of the adder 14 serves as a reference input to the inverter 4. The output 13 of the speed controller 12 instructs the slip frequency of the induction motor 5, and is controlled depending upon the load. Further, the output 13 of the speed controller 12 is limited by a predetermined saturation value of the speed controller 12.
The conventional device for driving the induction motor has been constructed as described above. Therefore, the induction motor produces different torques even for the same slip frequency depending upon whether it is running at low speeds or at high speeds. When the induction motor is running at low speeds such as during the regenerative running, excessively large torque is produced and heavy currents flow, resulting in the tripping of the device.